Yellowish edges appear in leaves deficient in

Options

A. potassium
B. calcium
C. magnesium
D. phosphorus.

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the nutrient deficiency that causes yellowish edges on leaves. This condition is known as chlorosis.

2. **Identifying Essential Nutrients**: We need to consider the essential nutrients for plants and their roles. The main nutrients that can cause chlorosis when deficient include potassium, calcium, magnesium, and phosphorus.

3. **Analyzing Potassium**: Potassium is essential for various plant functions, including the regulation of stomatal opening and closing. A deficiency in potassium can lead to a decrease in chlorophyll concentration in leaves.

4. **Linking Potassium Deficiency to Chlorosis**: When potassium is deficient, the chlorophyll content in leaves decreases, leading to yellowing of the leaf edges. This yellowing is referred to as chlorosis.

5. **Considering Other Nutrients**: While calcium, magnesium, and phosphorus are also important, they do not specifically cause yellowish edges in leaves as potassium does. Each of these nutrients has different roles in plant health.

6. **Conclusion**: Based on the analysis, potassium deficiency is the primary cause of yellowish edges appearing in leaves.

### Final Answer:
Yellowish edges appear in leaves deficient in **potassium**.

---